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Re: 1956 to 1960 California Sierra Snow Loads

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Your method to back out from the given member sizes is a sound approach,
if you plan on monitoring the snow depth that closely.

At some point, however, you will want to know the magntitude of the snow
load.  The building code mandates that this load be given to you by the
building official.  This would be the building official from whatever
city or county jurisdiction you're in.

In the absence of this being done, there are ways to figure it out. Do
this and coordinate the effort with the building official.

The first step is to contact the USGS, and get the best elevation above
msl for the site.

Also you should contact the Soil Conservation Service, and the National
Weather Service. They gather and record precipitaiton and snow depth
data.  You may be able to find maximum snow depth data from them at
their stations since they began keeping records.

A good rule of thumb to follow is that used by older addtions of the
Canada Building code, that is that 1" of snow is equal to 1psf, based on
snow having a specific gravity of .192.  This can be used as a benchmark
in your loads analysis.

Building code values for lumber can be found from codes during that
period.  Another useful tool is to use the allowable plank loadings in
the Western Woods Use Book  The thickness can be checked in the
installed condition using a small drill bit.  Or, older documents should
be able to give you the appropriate dimensions.

More useful information that the SCS and the NWS can provide you with is
the normalized ground snow isoline values.  The ground snowload is the
isoline value multiplied by the elevation in feet of the site under
consideration.

If you have or can get the isoline data the procedure is as follows:
1. Locate the site, and find the coefficient that is closest to the
site.
2. Multiply the isoline value by the site elevation number is feet.
3. Check if this value corresponds with any available data and adjust.

Make sure that you also check for sliding snow and drifting snow.
Depending on who you talk to, it is generally felt that for drifting
snow, the UBC and the ASCE document, overestimate density.

The SCS and the NWS should be able to point you in the right direction
for snow data. You should be able to find the appropriate documents on
line.  Also, you should be able to contact each of the western states
Structural Engineer's Association on line, and see what documents are
available for snow loading.

To determine the maximum alllowable height use the specific gravity of
.192

The Univerity of Idaho did a study in 1986 called Ground and Roof Snow
Loads for Idaho.  There is a depth load relationship looked at in this
document.  There may be subscribers that know more about that document.

Other good document to pursue as well:
Snow Loads for Structural Design in Montana
Snow Load Analysis for Washington (also for Utah and Oregon)

You might also check with the ASCE minimum design loads for the late
50's and early 60's.

Also, a certain percentage of the snow load must be included in the
seismic design of the structure.  This has to do with the probability of
a seismic event as well as the mean recurrence interval for whatever
snow load you end up with.  But it needs to be addressed in some form.

Hope this helps,

Bart Needham, SE


Robert_DuPlaine(--nospam--at)dot.ca.gov wrote:

> We are analyzing numerous maintenance buildings in the
> Sierras to determine when it becomes necessary to
> remove snow from the roofs ( maximum depth of snow ).
>
> The As-Builts do not include the design snow loading used
> or the required material properties. Must have been in the Specs.
>
> The Counties have no historical information on the
> required snow loads from the late 50's and early 60's.
>
> Typically, the roofs consist of built-up roofing over
> 2x  tongue and groove planking supported on steel trusses.
>
>    What material properties are appropriate for DF #2 T&G
>    of that vintage ???
>
>    Were  2x T&G  planks thicker in 1960  ???
>
>    Deflection typically controls ....
>    Can we push the TL deflection limits past L/180 ???
>
> Thank you for your assistance.
>
> Robert du Plaine   ( robert_duplaine(--nospam--at)dot.ca.gov )
> California Department of Transportation
>
> We are currently trying to back it out from the roof
> members and assumed strengths of materials
>
> Can anyone shed some light on this problem ???
>
> We are trying to determine the maximum allowable height
> of snow on buildings in the Sierras
>